1. Field of the Invention
The present invention relates to a flicker correction method and device, and an image pickup device, in which a flicker is corrected by subtracting a flicker correction signal from an image signal.
2. Description of the Related Art
When an object is imaged with a digital camera under the light of a light source that repeats turning on and off cyclically, such as a fluorescent lamp, cyclic light and dark fringes will appear in a captured image of the object, resulting in that they will seemingly run in the image. Otherwise, there will cyclically take place a difference in brightness between frames over an image. This is called “flicker”. The flicker is a problem unavoidably taking place when an object is imaged with a digital camera using an image sensor to image the object under the light of a flickering light source with the timing of charge storage being shifted.
In the conventional image sensor, the timing of charge storage varies depending upon whether the charge storage is made per plane or per line. Generally, timing of the charge storage per plane is called “global shutter system” while timing of the charge storage per line is called “rolling shutter system”. Most of the CCDs have adopted an image sensor of the global shutter type in the past. Recently, however, increasingly more attention has been paid to the CMOS image sensors that consume less power than the CCDs and can be produced more inexpensively than the CCDs because of their smaller number of parts. Many of the CMOS image sensors adopt the rolling shutter system for their structural problem. With one of the two shutter systems, when imaging is made under the light of a light source repeating turning on and off, light and dark fringes will appear in an entire image plane (plane flicker), while with the other shutter system, light and dark fringes will appear per line (in-plane flicker).
FIG. 1 shows a difference in amount of charge storage in an image sensor adopting the global shutter system, and FIG. 2 shows an example of image incurring a plane flicker when the image sensor is of the global shutter type. FIG. 3 shows a difference in amount of charge storage in an image sensor adopting the rolling shutter system, and FIG. 4 shows an example of image incurring an in-plane flicker. Also, a flicker component included in an image captured under the light of a light source cyclically turning on and off can be approximated to a sinusoidal wave, and there has been prevalent the method of forming a corrected image by removing the flicker on the basis of the nature of the sinusoidal wave.
For the flicker correction, there was proposed a method in which a flicker component is detected in an input image and the gain on the basis of a flicker component detected in an input image (as in the Japanese Patent Application Laid Open No. 2004-222228).